1. Field of the Invention
The present invention relates to an image processing apparatus having a recovery circuit for recovering bi-level pseudo-half tone image data, including bi-level image data obtained with a pseudo-half tone binarizing method or with an ordinary binarizing method, to a multi-level image data.
2. Description of the Prior Art
Usually, bi-level printers which reproduce a bi-level image data are used. Further, recently, there has been put into practice a color laser printer for recording an image of multi-level image data at a high speed and with a high resolution.
Multi-level data of a half-tone image such as a photograph are expressed as multi-level data of a plurality of bits per pixel. However, a large capacity of memory is needed to store such a multi-level image data.
In order to solve the above problem, the following method is suggested. Namely, the multi-level image data are converted into bi-level image data, and then, the converted bi-level image data are stored in the storage unit. Furthermore, upon processing the bi-level image data or recording the bi-level image data, after reading out the above bi-level image data from the storage unit, multi-level image data are recovered from the bi-level image data.
This type of recovery of bi-level data of pseudo-half-tone image to multi-level image data has an advantage that when a pseudo-half-tone image is reproduced by a multi-level printer or a multi-level display, the recovered multi-level image data can make use of the gradation output of the output apparatus effectively.
Further, the recovery has another advantage. That is, when a pseudo-half-tone bi-level data is outputted (recorded or displayed) at a different pixel density, magnification change is performed not with a simple magnification processing, but after the recovery to multi-level data. This can prevent the generation of Moire patterns due to the periodicity of the original pseudo-half-tone bi-level data. The recovered multi-level data is processed with a pseudo-half-tone method and then is sent to an output apparatus. Then, if the output system has a higher density than the original image data, the high density characteristic of the output system can be used effectively.
Further, the two advantages can be combined to output multi-level data after the magnification is changed.
The method and apparatus of this type are disclosed in the Japanese patent laid-open publications Nos. 62-114378/1987 and 62-107573/1987.
The image processing method disclosed in the Japanese patent laid-open publications No. 62-114378/1987 intends to make the best use of a multi-level printer even for bi-level image data as well as to improve the quality of recovered character image and includes the following steps: (a) recovering a half-tone image from the inputted bi-level image data; and (b) performing predetermined processes such as a magnification and reduction process, an image emphasis process or the like for the recovered half-tone image. In this method, in order to recover multi-level image data from bi-level image data, a window in a square shape having a predetermined size is provided close to a specified pixel to be recovered, and then, a smoothing process is performed for the inputted image data within the provided window.
Further, the image processing apparatus disclosed in Japanese patent laid-open publications No. 62-107573/1987 intends to prevent the quality of image from deteriorating in the case of using a binarizing circuit having a simple structure using a dither method or the like and also to constitute the image processing apparatus with simple circuits and comprises the following means: (a) dividing means for dividing binarized image information into predetermined blocks; (b) judging means for judging the image tone by each block; and (c) converting means for converting the binarized image information within the block into multi-level image data according to the judgment result thereof. In this apparatus, upon transmitting image data and storing them, there are used bi-level image data so as to display and edit them more efficiently. Further, upon reproducing the image, there is used a multi-level representation close to an analogue image. Concretely, the judgment means is constituted by an image judgment table ROM for judging the image tone using a pattern matching method within each block corresponding to a size of a dither matrix, and the converting means is constituted by a conversion ROM.
When a multi-level image is recovered from a pseudo-half-tone bi-level image, the mixing of a smoothing component obtained by a window for smoothing and an edge emphasis component obtained by a window for edge emphasis on pixels around a specified pixel to be processed can be used.
In this case, when an edge emphasis component is obtained as to a direction, an edge component (edge emphasis amount) is detected from a difference of the numbers of pixels in two pixel blocks (windows). However, an edge may be detected erroneously in a different direction if a pixel which is relatively far from the specified pixel is included in a window for edge emphasis, or especially if a separate pixel exists in a direction different from the direction for edge emphasis. Therefore, an image which should be smoothed may be emphasized or a negative emphasis may be emphasized positively, so that correct recovery of an original multi-level image is not realized.
Though a half-tone image is usually an image of frequencies lower than a period of two pixels, an original image of a straight line of width of one pixel may be binarized with a pseudo-half tone binarizing method. In this case, if a window of width larger than one pixel in the direction of edge emphasis is used, the original image may become a dashed line of width of one pixel or more or the density of the bi-level image may disperse. Thus, an edge emphasis component cannot be obtained and the original image cannot be recovered.
A window of wider width is favorable in order to get an image smoothed more, while a window of narrower width is needed in order to get an edge emphasis component of an image of higher frequencies. However, if the sizes of the windows for detecting a smoothing component and an edge emphasis component are different from each other, the effect of the larger window becomes larger.